Commodity accepted by general consent as a medium of economic exchange. It is the medium in which prices and values are expressed; it circulates from person to person and country to country, thus facilitating trade. Throughout history various commodities have been used as money, including seashells, beads, and cattle, but since the 17th century the most common forms have been metal coins, paper notes, and bookkeeping entries. In standard economic theory, money is held to have four functions: to serve as a medium of exchange universally accepted in return for goods and services; to act as a measure of value, making possible the operation of the price system and the calculation of cost, profit, and loss; to serve as a standard of deferred payments, the unit in which loans are made and future transactions are fixed; and to provide a means of storing wealth not immediately required for use. Metals, especially gold and silver, have been used for money for at least 4,000 years; standardized coins have been minted for perhaps 2,600 years. In the late 18th and early 19th century, banks began to issue notes redeemable in gold or silver, which became the principal money of industrial economies. Temporarily during World War I and permanently from the 1930s, most nations abandoned the gold standard. To most individuals today, money consists of coins, notes, and bank deposits. In terms of the economy, however, the total money supply is several times as large as the sum total of individual money holdings so defined, since most of the deposits placed in banks are loaned out, thus multiplying the money supply several times over. See alsosoft money.